Merck Manual

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Poison Ivy Dermatitis

Poison Ivy Dermatitis

About 50 to 70% of people are sensitive to the plant oil urushiol contained in poison ivy, poison oak, and poison sumac. Similar oils are also present in the shells of cashew nuts; the leaves, sap, and fruit skin of the mango; and Japanese lacquer. Once a person has been sensitized by contact with these oils, subsequent exposure causes a contact dermatitis.

The oils are quickly absorbed into the skin but may remain active on clothing, tools, and pet fur for long periods of time. Smoke from burning plants also contains the oil and may cause a reaction in certain people.

Symptoms of poison ivy dermatitis begin 8 to 48 hours after contact and consist of intense itching, a red rash, and multiple blisters, which may be tiny or very large. Typically, the blisters occur in a straight line following the track where the plant brushed along the skin. The rash may appear at different times in different locations either because of repeat contact with contaminated clothing and other objects or because some parts of the skin are more sensitive than others. The blister fluid itself is not contagious. The itching and rash last for 2 to 3 weeks.

Recognition and avoidance of contact with the plants is the best prevention. A number of commercial barrier creams and lotions can be applied before exposure to minimize, but not completely prevent, absorption of oil by the skin. The oil can soak through latex rubber gloves. Washing the skin with soap and water prevents absorption of the oil if done immediately. Stronger solvents, such as acetone, alcohol, and various commercial products, are probably no more effective. Desensitization with various shots or pills or by eating poison ivy leaves is not effective.

Treatment of poison ivy dermatitis helps relieve symptoms but does not shorten the duration of the rash. The most effective treatment is with corticosteroids. Small areas of rash are treated with strong topical corticosteroids (drugs applied to the skin), such as triamcinolone, clobetasol, or diflorasone—except on the face and genitals, where only mild corticosteroids, such as 1% hydrocortisone, should be applied. People with large areas of rash or significant facial swelling are given high-dose corticosteroids taken by mouth. Cool compresses wet with water or aluminum acetate may be used on large blistered areas. Antihistamines given by mouth may help with itching. Lotions and creams containing antihistamines are seldom used.